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Soil pH management

Soil pH governs nutrient availability and microbial activity. Most field crops perform optimally between pH 6.0 and 7.5, where macronutrient release is highest and toxic-ion effects are minimal. Outside this range, amendments are used to bring pH into the favourable window or to displace problem ions; the appropriate material and dose depend on the soil's existing chemistry.

Identification

Acid soils, with pH below 6.0, are common in high-rainfall belts and are diagnosed by low pH and often by aluminium or manganese toxicity symptoms in crops. Alkaline soils with pH above 8.5 are common in semi-arid plains and may be sodic, with high exchangeable sodium that disperses soil aggregates and impairs infiltration. Routine Soil Testing reports pH along with EC and organic carbon, providing the starting point for amendment decisions.

Management

Acid soils are corrected with agricultural lime (calcium carbonate or dolomite), which adds calcium and magnesium and neutralises hydrogen and aluminium toxicity. The reaction is slow, taking 2-3 years to fully equilibrate after a single application. Alkaline and sodic soils are reclaimed with elemental sulphur, sulphuric-acid by-products or gypsum: gypsum supplies calcium that displaces exchangeable sodium and improves structure, but does not by itself lower pH; sulphur and acid-forming materials do. Ammonium sulphate and single super-phosphate (SSP) are acid-forming fertiliser options that help acidify high-pH plots over time. Reclamation of saline-sodic paddy fields is covered in detail under Saline Soil Paddy Management.

Adoption context

Lime is widely used on acid soils of the eastern plateau, north-east hills and parts of the Western Ghats. Gypsum-based reclamation is supported by ICAR-CSSRI in the Indo-Gangetic plain. Correct pH improves availability of phosphorus, zinc and other micronutrients, addressing one route to Hidden Hunger Micronutrients and Zinc Deficiency Paddy.

Limitations

Amendments are bulky and transport cost can dominate field cost. Over-liming acid soils can induce micronutrient deficiencies; over-application of sulphur on alkaline soils can release toxic levels of soluble salts. A soil test is mandatory before deciding the amendment dose.

See also: Soil Testing, Soil Organic Carbon Management, Saline Soil Paddy Management, Hidden Hunger Micronutrients, Zinc Deficiency Paddy.

References

  1. Amending Soils with Lime or Gypsum (NRCS 333). USDA Natural Resources Conservation Service.
  2. Facts about Soil Acidity and Lime. Michigan State University Extension.
  3. Soil Quality Agronomy Technical Note 8. USDA NRCS.